Piezoelectric ceramic compositions principally containing lead zirconate titanate (Pb(Ti, Zr)O3, hereinafter referred to as “PZT”) have excellent piezoelectric properties and therefore are widely used for piezoelectric devices such as piezoelectric actuators.
In order to create large distortions, piezoelectric materials used for piezoelectric actuators need to have a large piezoelectric constant. Piezoelectric devices operated with high voltages are subject to increased temperature because of heat generation during operation. An increase in the dielectric constant of a piezoelectric material increases the amount of heat generated from a device made thereof. This is because the increase of the dielectric constant leads to an increase in capacitance to cause an increase in current. There is a problem in that piezoelectric properties of the device are significantly impaired and therefore the distortion thereof is decreased when the temperature of the device approaches the Curie point. Furthermore, there is a problem in that the occurrence of phase transition causes the depolarization of the piezoelectric material which can prevent the device from functioning as a piezoelectric actuator when the temperature of the device exceeds the Curie point. In order to continuously operate the piezoelectric actuators, the piezoelectric materials need to have a small dielectric constant and a high Curie point in addition to having a large piezoelectric constant.
Patent Documents 1 and 2 disclose piezoelectric ceramic compositions in which PZT is doped with Pb(Ni1/3Nb2/3) O3 (hereinafter referred to as “PNN”) or Pb(Zn1/3Nb2/3)O3 (hereinafter referred to as “PZN”) in order to enhance these properties.
In particular, Patent Document 1 discloses a solid-solution piezoelectric ceramic composition containing four components: PNN, PNZ, PZ and PT. In order to achieve a large piezoelectric constant, a large electromechanical coefficient, a large dielectric constant, and a high Curie point, the amount of Pb is adjusted to be less than its stoichiometric composition.
Patent Document 2 discloses a piezoelectric ceramic composition having the composition formula aPbTiO3-bPbZrO3-cPb((Zn1-xNix)2/3)O3, wherein a, b and c are each located in a predetermined region in the ternary diagram thereof. In this composition, piezoelectric properties and continuous operation durability are balanced. Pb in this composition is partially replaced with at least one of Ba, Sr and Ca.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 60-103079
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2003-335579